Fix zero time issue when updating time sheets

modules/time_sheets/lib.js

@@ -807,7 +807,7 @@ QtsheetAdapter.method('validateCellValue', function(element, evt, newValue) {
 		return false;
 	}
 	var val = parseFloat(newValue);
-	if(val <= 0  || val > 24){
+	if(val < 0  || val > 24){
 		return false;
 	}